If we want to create a riff for a song that doesn't fall into a standard chord scheme category, we need a little strategy. I usually break up the chord scheme in 2-bar sections and simplify the chords, so we get an idea of the basic structure without any substitute chords. First of all, if the riff is for accompaniment, you wouldn't necessarily need it throughout the whole chorus. You can take 2's: 2 bars riff, 2 bars solo, or play 4 by 4, or even take 8's.

Now we're going to look at the riff over the whole song just to get the practice, and in case you need the other odd bits later in the song. Have a look at the tab for the more or less official/usually played chords. It doesn't matter here if you agree with them or not. Here's how I'd break up a tune like "Indiana" into 2 bar bits that would fit a 2 bar riff.